You have a geometry, which stores a shape in Euclidean geometry, and you want to associate a point on the globe, represented by a latitude and longitude, with it, to see if it is inside. This won't work, due to the way SQL stores the data. You probably need to use Geography data types to check this - Latitude and longitude are points on a sphere (Actually geodetic data, since the earth is not quite a sphere.)
For more information on why they are different, see this explanation from microsoft. Also this answer on stackoverflow: GEOMETRY and GEOGRAPHY difference SQL Server 2008
To convert your data from geometry to geography, try:
Geography::STGeomFromText(cast(GeomCol as varchar(max)), 4326)
Then you can use the STIntersects method, documented by microsoft here.
